The present invention relates to limited current type oxygen sensors, and, more particularly those which can be employed effectively as oxygen deficiency monitors for use in automobiles and boilers.
U.S. patent application Ser. No. 008,655, filed Jan. 30, 1987 and assigned to the common assignee, Fuji Electric Co., Ltd., which application is hereby incorporates by reference, discloses an invention made by inventors including all the applicants of the present invention: a limited current type oxygen sensor formed with an oxygen ion conductive solid electrolyte of zirconia as shown in FIGS. 5(A) and 5(B) as represented generally by the numeral 100. The shape and area of a detection element 11 in oxygen sensor 100 are made equal to or slightly smaller than the shape and area of a space 3 in order to prevent electrical interference between detection element 11, in which ionic current flows between electrodes 1, 2 embedded in a solid electrolyte, and a heater element 12 with heaters 5 embedded in a solid electrolyte. Space 3, sandwiched between detection element 11 and heater element 12, communicates with a ssmall hole or diffusion orifice 4 for permitting ambient gas to enter the space 3. Leads 6, 7 of electrodes 1, 2 are connected to terminals 10A, 10D, respectively, whereas heater 5 is connected to terminals 10B, 10C, respectively.
In this oxygen sensor 100, the heater element 12 should be heated up high enough to overcome the thermal resistance caused by the space 3 disposed between the detection element 11 and the heater element 12 for heating the detection element 11 up to a predetermined temperature of operation of the sensor 100. Accordingly, as the amount of power required to operate sensor 100 tends to increase, the heater element 12 is heated to a high temperature to the detriment of long-term stability of the heater element 12. In addition to this disadvantage, thermal stress that occurs in the oxygen sensor 100 may ultimately damage the oxygen sensor 100 itself (as the temperature difference between the vicinity of the heater element 12 and the remaining portions of sensor 100 increases).